feat(dracut.sh): populate uefi_cmdline if no other cmdline is given

[pvalena]

feat(dracut.sh): populate uefi_cmdline if no other cmdline is given

When creating uefi image in hostonly mode (with using --hostonly-cmdline),
it makes sense to copy current kernel commandline from /proc/cmdline,
in case there are no other options specified.

Usually, the cmdline.d/*.conf file is generated by module rootfs-block
(or other modules), but it might, not handle all cases correctly,
and specifying --kernel-cmdline every time is not much user-friendly.

Changes

/proc/cmdline will be used as a last option for for uefi .cmdline entry.

Checklist

• [ ] I have tested it locally: TBD

• [ ] I have reviewed and updated any documentation if relevant TODO: document this

• [ ] I am providing new code and test(s) for it

Shouldn't be probably needed/desired to test this, as the host /proc/cmdline would need to be remounted.